Piezoelectric thin film resonators using Bulk Acoustic Wave (BAW), which are a type of acoustic wave device, are employed in filters and duplexers of wireless communication devices such as mobile phone terminals. The piezoelectric thin film resonator has a structure designed to have a lower electrode and an upper electrode facing each other across a piezoelectric film. The piezoelectric film typically has a negative temperature coefficient of elastic constant, and thus its resonant frequency and anti-resonant frequency change according to temperature. To reduce the change in frequency due to temperature, there has been suggested using a silicon oxide film having a temperature coefficient of elastic constant opposite in sign to that of the piezoelectric film as a temperature compensation film. For example, Japanese Patent Application Publication No. 2007-159123 suggests using a boron-doped silicon oxide film as a temperature compensation film. For example, Sang Uk Son and 6 others suggest using a multilayered film made of a Low Temperature Oxide (LTO) film and a silicon nitride film as a temperature compensation film in “Temperature Compensated Bulk Acoustic Wave Resonator (BAWR) for Near Zero Temperature Coefficient of Frequency (TCF)”, Proceedings of the 42nd European Microwave Conference, October-November, 2012, p. 100-103.
In addition, there has been known providing a silicon nitride film on a silicon dioxide film in a Surface Acoustic Wave (SAW) device, which is a type of acoustic wave device, to prevent moisture absorption of the silicon dioxide film covering a comb-shaped electrode as disclosed in Japanese Patent Application Publication No. 2011-61743. In addition, there has been known providing a silicon nitride film covering a comb-shaped electrode in a SAW device to prevent moisture absorption of the comb-shaped electrode as disclosed in Japanese Patent Application Publication No. 2003-198321.
In the piezoelectric thin film resonator, a silicon oxide film used as a temperature compensation film may be located at the opposite side of at least one of the lower electrode and the upper electrode from the piezoelectric film. When the silicon oxide film, which is located at the opposite side of at least one of the lower electrode and the upper electrode from the piezoelectric film, makes contact with the lower electrode and the upper electrode, the lower electrode and the upper electrode are oxidized by oxygen contained in the silicon oxide film through, for example, a high-temperature reliability test. Moreover, since the silicon oxide film exhibits hygroscopic properties, it absorbs water and causes a change in film quality through, for example, a high-temperature reliability test. These are the causes of the deterioration of the characteristics of the resonator.